Honors Chemistry
Name: ____________________________
Period: _____
Date: _____________
Lab Partners: ______________________________________________________
Observing a Chemical Reaction
Purpose:
To learn how qualitative and quantitative observations of a chemical reaction are used to
formulate a hypothesis. To be able to distinguish between a chemical change and a physical
change.
Introduction:
You and a friend may have very different feelings about a movie you’ve just seen. You may
disagree about whether you liked the movie or about the movie’s intended meaning. Although
you both have observed the same movie, your interpretations of the movie may differ.
Distinguishing between observation and interpretation is very important in chemistry. An
observation is a statement of fact, based on what you detect by your senses. An interpretation is
your judgment or opinion about what you have observed. A statement such as “the liquid is clear
and colorless” is an observation. It would be an interpretation to say, without further testing, that
the clear and colorless liquid is water.
The purpose of this experiment is to help you distinguish observation from interpretation while
examining a chemical reaction. Try to make as many observations of the reaction as possible.
There are two types of observations. A quantitative observation is an observation that involves a
measurement; a qualitative observation is a general description and does not involve a
measurement. “The liquid is hot” is a qualitative observation. “The temperature of the liquid is
95°C” is a quantitative observation.
During a chemical change, the composition of matter changes and the change is not reversible by
simple physical means. The chemical reaction will start with a reactant and produce a new
product. Possible clues to chemical change are a transfer of energy, a permanent change in
color, the production of a gas, the formation of a precipitate, and the permanent change in texture
and shape. Some of these changes are similar to physical changes, but physical changes are
more reversible except for cutting, grinding, or cracking.
Material:
 Safety goggles & apron
 150 mL beaker
 Glass stirring rod
 Thermometer
 Copper (II) chloride dihydrate (CuCl2•2H2O)
 Distilled water
 Weighing dish
 Aluminum foil, 8cm x 8cm
Safety Precautions:
 Wear safety goggles and an apron at all times.
 Copper (II) chloride is an irritant. Avoid skin contact with copper (II) chloride.
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Honors Chemistry

Observing a Chemical Reaction
Copper (II) chloride is toxic by ingestion and inhalation.
Pre-Lab Questions:
1. What is a physical change? Give an example.
2. What is a chemical change? Give an example.
3. What are possible clues you have had a chemical reaction?
Procedure:
As you perform the experiment, record your observations in the Data Table.
1. Obtain and describe a sample of copper (II) chloride dihydrate.
2. Add 25 mL of distilled water to the 150 mL beaker. Mass approximately 5.000 g of
CuCl2•2H2O and record the mass. Without stirring, add the CuCl2•2H2O to the water.
Record your observations of both the solid and the water.
3. Use a glass stirring rod to stir the mixture until the solid is completely dissolved. Record
your observations of the solution.
4. Place the thermometer in the copper (II) chloride solution and record the temperature.
Caution: Observe the mixture from the side, do not look directly down into the beaker.
5. Mass the aluminum foil and record the mass. Place a loosely crumpled ball of aluminum
in the solution and record your observations. Stir the mixture occasionally until the
reaction is complete and observe what happens during the reaction. Let the beaker stand
without stirring and observe for at least 10 minutes. Record any change in temperature.
6. Liquids are to be poured down the sink drain and flushed with water. The solids are to be
disposed in the trash.
Observations:
System
Observations
Mass CuCl2•2H2O
Dry copper (II) chloride dihydrate
Copper (II) chloride in water
Stirred copper (II) chloride in water
Mass of aluminum
Copper (II) chloride solution plus
aluminum foil
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Honors Chemistry
Observing a Chemical Reaction
System
Observations
Initial temperature:
Final temperature:
Analyses and Conclusions:
1. Check your observations. Cross out any that are interpretations rather than observations.
2. List each of your observations in one of the four following sections. Number your
observations consecutively. Circle the number of any observation that is quantitative.
a. Observations of the dry solid
b. Observations of the wet solid before stirring
c. Observations of the solution before addition of aluminum
d. Observations of the reaction that occurs when the aluminum is added
3. Would you expect your observations or your interpretations to most closely match those
of your classmates? Explain.
4. Was the dissolving of the copper (II) chloride dihydrate a physical or a chemical change?
Explain your reasons.
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Honors Chemistry
Observing a Chemical Reaction
5. Was the addition of aluminum to the solution a physical or chemical change? Explain
your reasons.
Going Further:
Develop a Hypothesis:
Based on the observations you made during this lab, develop a hypothesis about what happened
when aluminum metal was added to the solution of copper (II) chloride.
Design an Experiment
Propose an experiment to test your hypothesis. If resources are available and you have the
permission of your teacher, perform the experiment.
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Observing a Chemical Reaction
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